1. Field of the Invention
The present invention relates to a control system for automatic transmissions, and more particularly, to a control system for automotive automatic transmissions.
2. Description of the Prior Art
Japanese Patent First Provisional Publication 58-81258 shows a control system for a V-belt type continuously variable automatic transmission installed in a motor vehicle. In this control system, a so-called "fail-safe measure" is employed in which when a torque motor for a line pressure adjusting valve becomes out of order, the torque motor is deenergized and the line pressure adjusting valve is controlled to keep the line pressure at the highest level. With this measure, the motor vehicle is permitted to run irrespective of the trouble of the torque motor.
However, hitherto, some conventional control systems including the above-mentioned control system have failed to give sufficient consideration to an undesired short-circuit which might occur between the wiring of a solenoid for line pressure control and that of the other solenoids. In fact, the wires for the line pressure controlling solenoid and those for the other solenoids are bound into a harness. The harness is connected through a connector to another harness which extends from a control unit. Thus, in automatic transmission control systems having such wiring, an undesired short-circuit tends to occur between the neighboring wires due to, for example, breakage of sheaths of the wires, immersion of water through the broken sheaths, immersion of water into the connector or the like. Of course, the short-circuit of the wiring causes a trouble or stopping of the motor vehicle.
That is, upon the short-circuit of the wiring, the operation of the line pressure control solenoid becomes inconsistent with an instruction signal from the control unit. In this case, judging that there is an abnormal condition or malfunction in the control system, the control unit issues an OFF signal to the line pressure control solenoid. However, due to the short-circuit which is kept established, ON signals normally fed from the control unit to the other solenoids (such as, a solenoid for controlling the speed gear, a solenoid for controlling the lock-up mechanism, a solenoid for controlling the engine brake and the like) cause the line pressure control solenoid to become ON thereby establishing a lower line pressure. As is known, when the motor vehicle is continued to run with such lower line pressure fed to the transmission, the clutches and brakes of the transmission easily seize because of undertorque of the clutches and brakes.